1. Field of the Invention
The present invention relates in general to a nanostructured tungsten carbide material, and particularly to a nanostructured tungsten carbide-cobalt composite material and a method of fabricating the same.
2. Description of the Related Art
The synthesis and processing of nanophase or nanostructured materials, i.e. materials with grain sizes less than about 100 nm, is currently of great interest because these materials have properties different from and often superior to those of their conventional counterparts.
Nano-powders have been produced using synthesis methods such as electrodeposition, melt spun, plasma, chemical/physical vapor condensation, chemical precipitation, sol-gel and hydrothermal processings. Cemented tungsten carbide powders with nano-sized grains have been produced using the spray conversion process (SCP) disclosed in McCandlish U.S. Pat. No. 5,352,269 entitled xe2x80x9cSpray Conversion Process for the Production of Nanophase Composite Powdersxe2x80x9d. WCxe2x80x94Co materials are often used for cutting and drilling tool applications because of superior hardness and wear resistance. Nanostructured WCxe2x80x94Co materials have much better mechanical properties than their conventional counterparts. However, there exist problems with the consolidation of the nanostructured WC powders with retention of WC nano-grains. This is because the WC nano-grains grow rapidly during sintering.
It is an object of the present invention to provide a nanostructured tungsten carbide bulk material with ultrafine surface and good mechanical properties, such as high hardness, fracture toughness and wear resistance.
According to the present invention, the nanostructured tungsten carbide bulk material, sintered from tungsten carbide and metal such as cobalt nano-powders, comprises a tungsten carbide and a metallic binder such as cobalt phases. The sub-surface region of the tungsten carbide phase in the tungsten carbide/cobalt bulk has nanostructures comprising a plurality of dislocations, twins, stacking faults, dislocation cells, nano-subgrains with preferred orientation or texture, or a combination thereof.
It is another object of the present invention to provide a method of fabricating a nanostructured tungsten carbide bulk material.
The method comprises the following steps. First, the tungsten carbide and cobalt nano-powders were preferably mixed and milled with lubricants, binders, and grain growth inhibitors for a long time. The finer the powder, the more binders and lubricants as well as the longer time are needed. The milled powders were then pressed into the compacts. Following conventional sintering techniques for WCxe2x80x94Co, the compacts were first subjected to dewaxing and degassing in the presintering stage, followed by liquid phase sintering above a eutectic temperature for 60-100 min. Due to the large surface area of the tungsten carbide nano-powder, a relatively large amount of oxygen is absorbed onto the nano-powder. Therefore, in the presintering stage, the compacts were heated in flowing H2 atmosphere at about 800xcx9c900xc2x0 C. to remove the absorbed oxygen. During the sintering process, special care was taken to maintain the carbon-balance in the samples to prevent excess carbon from dissolving into the liquid cobalt during sintering. If excess carbon dissolves in cobalt during liquid phase sintering, phases such as W2C with poor mechanical properties can be formed. In the end of the process, the sintered compacts were sinter-HIPed with an argon pressure of 800 psi, for example, to further reduce porosity level in the sintered parts. Prior to ion implantation, sintered compacts were polished using diamond suspension (1 xcexcm particle size) to a mirror surface. Finally, ion implantation comprising Cxe2x88x92, N+, O2xe2x88x92, Ar+, or a combination thereof is preferably performed on the cemented tungsten carbide bulk at a pressure of 5*10xe2x88x924 psi below 100xc2x0 C., thus nanostructures are formed in the sub-surface region of the tungsten carbide phase, wherein the nanostructures comprise a plurality of dislocations, twins, stacking faults, dislocation cells, nano-subgrains with preferred orientation or texture, or a combination thereof. The dosage of the ion implantation is in the range of 1015 to 1030 ions/cm2.